1. Technical Field
The present invention generally relates to the technical field of gas and liquid contact devices and methods to produce an aerosol, and, more particularly, to an atomizer for use in aerosol generation at continuous, but ultra-low liquid flow rates such as 50 microliters per hour. Accordingly, the device of the present invention is referred to herein as a micro atomizer.
2. Description of the Related Art
Research in inhalation toxicology requires the ability to generate stable aerosol concentrations of a respirable particle size in a consistent manner from day to day and experiment to experiment. Respirable, as used herein, refers to airborne particles capable of entering and depositing in the upper airways and the lower airways of the lungs. The upper airways include the trachea and the primary, secondary, and tertiary bronchus. The lower airways include the bronchioles, terminal bronchioles, and alveoli. Aerosol deposition in the upper airways of the human is generally achieved by a particle size between about 2.5 and 10 microns (μm) aerosol mass median aerodynamic diameter (MMAD) with the larger particles depositing in the larger airways. Aerosol deposition in the lower airways of the human is generally achieved between 0.5 and 2.5 μm MMAD with maximal alveolar deposition occurring with particles of about 1.2 μm MMAD or smaller. For inhalation studies using small research animals, the range for the upper airways is approximately 2.0 to 6.0 μm MMAD and for the lower airways 0.5 to 2.0 μm MMAD.
In addition to inhalation toxicology, other research areas in the life and physical sciences, various manufacturing processes, and certain systems or products have a need for precise and accurate atomization to produce aerosols having small particle size at low to ultra-low, yet stable, concentrations. Various methods of generating such aerosols are in use, but suffer from a number of limitations, some of which are described below. These methods depend on parameters such as the physical state of the material to be aerosolized, vapor pressure of the material to be aerosolized and physical parameters of the desired aerosol exposure to be achieved.
While several commercially available products exist for liquid aerosol generation, for the inhalation toxicology research that motivated the present invention, none were found that would function properly and consistently at the required ultra-low liquid flow rates ranging from microliters per minute to microliters per hour. Additionally, the materials used in the studies for which the micro atomizer herein was invented require that the atomizer be constructed of materials resistant to organic solvents and caustic solutions. These features also would be desirable or required for most of the other applications mentioned above.
By way of example, one prior art device and method to generate aerosol at the desired liquid flow rates utilizes a modified commercially available spray atomizer nozzle (Spraying Systems Co., pneumatic atomizer model 1/4 JSS). This atomizer nozzle was modified to improve its performance for the inhalation toxicology studies by changing the delivery means of the liquid to be aerosolized. In particular, the reservoir injection means was replaced with a syringe and a syringe drive to more precisely control liquid flow at low flow rates. In addition, the liquid stream was kept separate from the atomizing air stream until reaching the tip of the nozzle. These modifications allow for greater control over the flow of liquid in comparison to the unmodified, commercially available pneumatic atomizer. Through use of this modified device, both aerosol particle size and aerosol concentration can be adjusted by changing either the liquid flow rate through adjustment of the syringe drive or by changing the air flow rate and pressure using a commercially available mass flow controller or pressure regulator.
Despite these benefits, the modified, commercially available pneumatic atomizer suffers operational limitations as it is a relatively large piece of equipment requiring liquid flow rates in the milliliter per minute/hour range as well as an air flow rate on the order of 20 liters per minute. These parameters make this generation method appropriate when relatively large volumes of liquid are available and acceptable, high aerosol concentrations are desired, and the application involves in a relatively large aerosol chamber. The chambers used in the inhalation toxicology studies that motivated this invention, however, are small in comparison and cannot operate with a generation system that outputs more than 5 liters per minute of air.
A second prior art device used to generate aerosol at the desired liquid flow rates involves the use of what may be referred to as a double-needle atomizer. This device involves the use of two small pieces of stainless steel tubing, or two appropriately sized needles such that the smaller of the two needles fits within the larger with enough room or clearance that air can flow through the larger needle. These needles are connected to a manifold that allows air to enter the larger needle and liquid to enter the smaller needle. The smaller needle carrying the liquid to be aerosolized is connected to a syringe, which in turn is connected to a syringe drive. The tip of the smaller needle carrying the liquid is set to extend approximately 0.5 mm past the tip of the larger needle carrying pressurized air. When an air flow through the larger needle is of appropriate pressure and flow rate, the liquid exiting the smaller needle is aerosolized. By way of example, the smaller needle in this prior art device is typically a 32 gauge blunt ended needle and the larger needle is a 21 gauge blunt ended needle.
Limitations of this second prior art device involve its inconsistency and unreliability in generating the same aerosol characteristics from day to day. While the double-needle design is capable of working in the desired microliter per minute to microliter per hour range and generates low concentrations in small aerosol chambers, its design has inherent flaws that cause it to perform in an inconsistent manner. In particular, the liquid-gas contact space is variable in day-to-day operations. This can occur, for example, due to small changes in the location and orientation of the inner needle relative to the outer needle. Such changes in the shape of the liquid-gas contact space result in undesirable changes in aerosol characteristics.
Another prior art method involves the adaption of commercially available ink jet cartridges. While ink jet cartridges can be utilized to produce aerosols, the amount of liquid needed exceeds the desired flow rates for the applications described above.
Accordingly, there remains a need for a micro atomizer capable of generating aerosols at liquid flow rates in the microliter per minute range or lower in a reproducible and consistent manner. Such a device can be used for the inhalation toxicology studies that motivated this invention as well as other applications in the life and physical sciences, manufacturing, and systems or products that require or could benefit from a micro atomizer as described herein.